1. Field of the Invention
The present invention relates to the manufacture of photographic masks used in the mass production of printed circuit boards.
2. Description of the Related Art
The most common method for producing printed circuit boards is by photoetching. In this process, a printed circuit board is produced from a phenolic board, or other suitable electrically insulating substrate material, having a copper foil layer on one or both sides of the board. Printed circuit lines, component connection pads and other features are formed by selectively removing portions of the copper foil so that only the desired lines and pads remain. The copper foil is removed by exposing the foil to an etchant solution. The portions of the copper that are to remain on the board after etching are protected by a layer of etchant resistant material that generally conforms to the lines and pads to be formed. The etchant resistant material is referred to as a "photo resist."
Although the photo resist could be applied to the copper foil only where desired, typically, during the mass production of printed circuit boards, the entire copper foil layer is first coated with the photo resist. Thereafter, the undesired portions of the photo resist are removed so that only the portions of the photo resist that conform to the desired lines and pads remain. This is accomplished by selectively exposing the photo resist to light so that, for example, the portions to remain on the copper layer are exposed to light and the undesired portions are removed. The photo resist is photosensitive. That is, when the photo resist is exposed to light of appropriate wavelength (e.g., ultraviolet light) and intensity, the photo resist changes its characteristics. For example, the exposure of some photo resist materials to light makes the photo resist insoluble to a developing solution so that when the printed circuit board is immersed in the developing solution, the unexposed portions of the photo resist are dissolved, leaving only the portions that were exposed to light. Other photo resist materials respond to the light in the opposite manner such that the portions exposed to light are soluble and the unexposed portions remain after immersion in the developer.
Alternative printed circuit board processes use the photo resist as a plating mask. That is the dissolve portions of the photo resist correspond to the lines and pads of the printed circuit board. A layer of tin/lead or other protective metal is plated onto the copper at the areas where the photo resist has been removed. Thereafter, the remaining photo resist is removed. The tin/lead plated areas are protected during the etching process, whereas the unplated areas are etched away. In addition, the tin/lead plating protects the resulting copper lines and pads during the remaining manufacturing processes and makes it easier to solder components to the printed circuit boards.
In order to control the exposure of the etchant material to the light, a mask is used. The mask is positioned between the printed circuit board and a light source. The mask has patterns of opaque areas that correspond to the portions of the etchant material that are not to be exposed to light, and the remaining areas are transparent to allow the light to expose the underlying etchant resistant material. Masks can have either positive patterns where the lines and pads are represented by opaque areas or negative patterns where the lines and pads are represented by the transparent areas. The choice of either positive or negative patterns is determined by the type of etchant material used or further processing to be performed during the manufacture of the printed circuit board.
As set forth above, a mask is similar to a transparency in that it comprises opaque and transparent sections which regulate the position and amount of light that can pass from a light source to the photo resist on a printed circuit board. The mask transparency, which can be either a photopositive or a photonegative, as discussed above, can be constructed in many ways. For example, until recently masks were constructed by draftsmen using layout tape. Typically, a draftsman would create a representation of the mask that was two or more times as large as the final mask transparency. This representation was photographed to produce a mask transparency of the proper size. It can readily be understood that this was inherently a time-consuming and inaccurate process.
More recently, the production of masks for printed circuit boards has been automated using computer aided design (CAD) to determine the placements of connection pads and other features on printed circuit boards and the routing of lines between the pads and other features. The output of the CAD system is routed to a laser photoplotter that generates a photopositive or photonegative image directly onto a mask transparency film by selectively exposing the photosensitive surface of the mask transparency film to a beam of light from a laser light source. The laser photoplotter operates in a manner similar to a television in that the laser beam is caused to scan across the surface of the mask transparency film in a raster scan pattern (e.g., across the film horizontally for each line and vertically from line to line). As the laser beam is scanned, the laser is turned on and off to selectively expose the mask transparency film to create the representations of the lines, pads and other features.
Since the laser photoplotters basically operate on a pixel format similar to the pixels used to create images on a video monitor and similar to the pixels used to create images on a laser printer, or the like, the resulting mask transparency can be considered to comprise a plurality of dots of exposed film. Depending upon the resolution of the laser photoplotter, the dots for horizontal and vertical lines will appear as continuous lines of uniform width. On the other hand, diagonal lines and circular features may appear jagged when viewed under a microscope.
A further characteristic of laser photoplotters relates to the construction of composite features such as the intersection between two lines, or the like. It should be understood that the databases used in the creation of printed circuit board masks were developed when vector photoplotters were the dominant systems for generating mask transparencies. Such vector photoplotters move a light source from point to point in order to create lines and other features. In addition, the widths of lines and the sizes of other features are controlled by changing the sizes and shapes of apertures between the light source and the mask transparency film and by changing the intensity of the light from the source. Thus, the vector photoplotter can be considered to be a continuous domain plotter in that virtually any pattern can be created on the mask transparency.
In order to be compatible with the pixel format of the laser photoplotter, the data from the vector format databases must be rasterized. That is, the individual lines, circles and other shapes generated for the vector photoplotters must be converted into a series of dots that represent the lines and other features. The dots correspond to pixels and the number of dots per unit area is limited by the resolution of the laser photoplotter. This conversion from the continuous domain of the vector photoplotter database to the discrete domain of the laser photoplotter inherently causes a laser photoplotter to have inaccuracies since the exposure by the laser beam is provided on a pixel by pixel basis. In particular, many of the lines and features to be plotted fail to correspond to the centers of the available pixels. Thus, the software that converts the vector database information to a raster database for plotting by a laser photoplotter must necessarily select pixels that best represent the features to be plotted. This is a particularly difficult problem when a horizontal or vertical line intersects with a line that is neither horizontal or vertical (e.g., a line at 45.degree.). The resulting intersections often exhibit bumps and shifting of the lines proximate to the intersection. Such bumps and shifting are generally unavoidable using conventional rasterizing software, particularly when the line widths are very small. A need exists for a method to reduce the inaccuracies in rasterizing intersecting lines for mask transparencies.